Re: v4.6 kernel BUG at mm/rmap.c:1101!

[Mika Westerberg]

On Mon, May 23, 2016 at 05:08:26PM +0200, Andrea Arcangeli wrote:
> On Mon, May 23, 2016 at 05:06:38PM +0300, Mika Westerberg wrote:
> > Hi,
> > 
> > After upgrading kernel of my desktop system from v4.6-rc7 to v4.6, I've
> > started seeing following:
> > 
> > [176611.093747] page:ffffea0000360000 count:1 mapcount:0 mapping:ffff880034d2e0a1 index:0x1f9b06600 compound_mapcount: 0
> > [176611.093751] flags: 0x3fff8000044079(locked|uptodate|dirty|lru|active|head|swapbacked)
> > [176611.093752] page dumped because: VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address))
> > [176611.093753] page->mem_cgroup:ffff88049e81b800
> > [176611.093765] ------------[ cut here ]------------
> 
> This is a splitted pmd tail that is triggering a COW, but it's still a
> compound page because the physical split didn't happen yet.
> 
> So like Kirill correctly pointed out, in such case we've to do
> compound_head because the page->mapping that has to be refiled to the
> local anon_vma is in the head.
> 
> It's just a false positive VM_BUG_ON, the code itself is correct.

OK, thanks for the explanation.

> Production kernels should be built with CONFIG_DEBUG_VM=n so this is
> not going to affect them and there's no bug for the production builds.

Hmm, the kernel shipped with Fedora 23 has that enabled:

lahna % grep CONFIG_DEBUG_VM /boot/config-4.4.9-300.fc23.x86_64 
CONFIG_DEBUG_VM=y
# CONFIG_DEBUG_VM_VMACACHE is not set
# CONFIG_DEBUG_VM_RB is not set

> Can you test this to shut off the false positive?

I'm testing with Kirill's patch (because he sent it first ;-)) and let
you know what happens. Thanks!